The purpose of the proposed research is to improve our understanding of social development. To do this, the infant-adult dyadic relationship will be studied to determine what properties of the adult's responses determine the infant's behavior at that point in time, and conversely. The social conditioning paradigm (baseline, stimulation, extinction) will be used as a framework for the analysis of social interaction. It will be determined whether, and in what ways, the infant is sensitive to correlational differences between response- dependent and response-independent social stimulation. The effects of the unresponsive-adult on vocalization rate, and on the effectiveness of subsequent social stimulation will studied to determine whether "baseline" procedures provide an appropriate comparison for the effects of social stimulation, and to determine whether adults can suppress, as well as facilitate infant social behavior. This research will have implications for the understanding of social, cognitive, and language development, and, will help to answer the parent who feels that infant social behavior (e.g., smiling, vocalizing, eye contact) is important and asks: "How can I encourage my baby to be more social?"